Dysfunction of the oral muscles presents itself in many forms and occurs with alarming frequency in the population. Most harmful oral habits occur on a subconscious level or during sleep. Treating incorrect behavior of the muscles of the tongue and the muscles of mastication are of interest to dentists, physicians, and speech therapists.
Tongue function. The proper rest posture for the tongue is flat against the palate (roof of the mouth). In the growing child, the lateral growth of the maxilla is stunted when the tongue does not rest in the palate. Cross bites, crowding and poor occlusion are commonly seen. During sleep, the tongue should maintain posture subconsciously against the palate. If the tongue fails to maintain contact against the palate, it can fall back to a position in the pharynx that obstructs respiration. This is one of the causes of snoring and sleep apnea. A person with significant structural or inflammatory nasal obstruction is obliged to breathe through the mouth. In this case, the tongue rests inferiorly on the floor of the mouth to allow the passage of air through the lips, over the tongue, and into the pharynx. Proper tongue rest position cannot be established with out medical relief of the nasal obstruction. Removal of the obstruction does not always change the tongue posture. Inferior tongue posture in these individuals seems to be habitual, or a behavioral error.
The swallow is a reflexive behavior, but can be consciously controlled. The correct swallowing motion for the tongue is performed with the lower posterior teeth touching the upper posterior teeth and the tongue remaining against the roof of the mouth. The pressure at the tip of the tongue is increased against the anterior palate and the contact area rolls posteriorly sweeping the bolus of food or liquid into the pharynx. A common error in swallowing occurs in persons who thrust their tongue anteriorly between the upper and lower front teeth. This is sometimes called a “tongue thrust” or “infantile swallow.” The repetitive forceful contact of the tongue against the incisal edges of the upper and lower anterior teeth produces an intrusive force against the incisors. The tongue thruster also fails to bring the lower posterior teeth against the upper posterior teeth, which results in excessive vertical growth of the posterior teeth. The result is an open bite malocclusion with no contact or vertical overlap of the upper and lower incisors. In some instances the tongue will thrust laterally instead of anteriorly, causing a lack of occlusal contact in one or both sides of the dentition. Successful treatment requires training correct swallowing behavior.
Proper speech requires the adept coordination of movement of the vocal chords, lips and tongue, with feedback from hearing. Many individuals have difficulty with correct tongue position and exhibit poor control and strength of the tongue. Therapeutic exercises are usually prescribed to correct the articulation errors.
Masticatory muscle loading. The muscles of mastication are normally active during chewing, and briefly during swallowing. Muscle contraction outside of these activities is considered to be dysfunctional. It is termed bruxism, clenching or grinding. This behavior is believed to be a response to stress, pain, and irregular occlusion. It causes pain, joint damage, dental attrition, and periodontal damage. Long-term bruxism also causes hypertrophy of the masticatory muscles and may, through intrusion of the posterior teeth, structurally reduce lower facial height. Most treatments of bruxism are designed to reduce the intensity of the muscle loading or shield the oral structures from the effects of non-physiologic forces.
Conversely, hypoactive masticatory muscles, that fail to load during swallowing, contribute to the creation of open bite malocclusion. In such individuals, the erupting lower and upper teeth often fail to meet in a balanced cusp-fossa relationship. The contacts between the upper and lower teeth will usually be few, and located mostly in the posterior regions. With time, the posterior teeth will be worn flat, requiring repair. The unchecked passive eruption of the posterior teeth causes increased lower facial height, and a dolicocephalic facial form. Current treatments may surgically improve the skeletal dimensions in such cases, but no treatment has yet been shown to strengthen and tone the masticatory muscles.